Effect of heat treatment on the structure, piezoelectricity and actuation behavior of a cellulose electroactive-paper actuator
Introduction
In recent years, polymers that can directly convert electrical energy into mechanical energy (electromechanical effects) have attracted much attention. The advantage of such polymeric materials is their soft biomimetic electromechanical properties, which have the potential to mimic biological responses. The polymers in this category recognized so far include ionic polymer metal composites (IPMCs), gel polymers, conductive polymers (polypyrrole and polyaniline) [1], [2] and piezoelectric polymers such as polyvinylidene fluoride (PVDF), copolymers of vinylidene fluoride and trifluoroethylene P (VDF-TriFE), polyvinyl fluoride, nylon and polyvinyl chloride (PVC) [3], [4], [5], [6], [7], and cellulose [8].
Cellulose is a natural linear polysaccharide in which D-glucopyranose rings are connected to one another via β-(1→4)-glycosidic links. Cellulose is the most abundant polymer found in nature, it is produced in a sustainable way and offers many possible uses, because it is renewable and biodegradable. Scientists have made cellulose into many types of films, such as films for hemodialysis [9], water resistance [10], sensors [11] and actuators [8]. Cellulose electroactive-paper (cellulose EAPap) actuators are made by depositing a thin gold electrode on both sides of a cellulose strip; the application of an electric field across the thickness of the strip then generates bending displacement. The working principle of cellulose EAPap is the combination of the piezoelectric effect as well as ionic effects associated with the dipole moment of the cellulose paper ingredients [12]. Cellulose and wood have a piezoelectricity due to their crystallinity. Many researchers have reported the more ordered structure [13], improved crystallinity [14] and piezoelectricity [15], [16] of wood cellulose after heat treatment.
In the present investigation, regenerated cellulose films were subjected to heat treatment in an attempt to improve the crystallinity. Following heat treatment for 2 h at different temperatures (60, 85, 105, 130, 150 and 170 °C), the changes in structure, direct piezoelectricity and actuation behavior were studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV–visible spectroscopy, piezoelectric constant (d31) measurement and tip displacement measurement.
Section snippets
Materials
Cotton cellulose (MVE, DPW 4580) was purchased from Buckeye Technologies Co., USA, N,N-dimethylacetamide (DMAc, anhydrous, 99.8%) from Sigma Aldrich, USA. Extra-pure lithium chloride was carefully dried with molecular sieves 1 week before use and DMAc was purchased from Junsei Chemicals Co., Japan.
Preparation of cellulose solution
Cotton cellulose pulp and LiCl were heated under reduced pressure at 110 °C for 1 h and the LiCl was dissolved in DMAc. Then, the pulp was mixed with LiCl–DMAc solution and heated at 155 °C, followed by
Sample weight
Weight of the cellulose films, before and after the heat treatment, were recorded and per cent weight loss was calculated bywhere W0 is weight of sample before heat treatment and WH is the weight of sample after heat treatment. The surfaces and cross-section of the films were examined with a scanning electron microscope (Hitachi S4300) to evaluate the effect of treatment on the morphology of the films. The films were coated with platinum under vacuum before the SEM
Weight loss of cellulose films
The per cent weight loss of the cellulose films after heat treatment for 2 h at 60, 85, 105, 130, 150 and 170 °C were 1.07, 1.61, 4.42, 4.85, 7.83 and 8.36%, respectively. The weight losses of the samples increased with increasing treatment temperature. However, the change in sample weight is not great and decreased to less than only 10% even after treatment at 170 °C for 2 h. The weight loss of cellulose in the temperature range 100–220 °C may be mainly due to physical loss of water and changes in
Conclusions
EAPap actuators were made by using regenerated cellulose films heat-treated at different temperatures from 60 to 170 °C for 2 h. A 10-fold increase in piezoelectric constant (d31) was observed after the sample had been heat-treated at 60 °C for 2 h compared to that of non-heat-treated film. However, when the treatment temperature was raised above 60 °C the piezoelectric coefficient was higher than that of the non-heat-treated film but lower than that of film treated at 60 °C. This may be due to the
Acknowledgment
This work was supported by the Creative Research Initiatives (EAPap Actuator) of KOSEF/MOST, South Korea.
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